Other files may be included in the mapping file. This is done with a line of the form:
This effectively substitutes the contents of the file file-spec into the mapping file at the point where the include appears. The file specification should specify a full file path (directory, and so forth). All files included in this fashion must be world readable. Comments are also allowed in such included mapping files. Includes can be nested up to three levels deep. Include files are loaded at the same time the mapping file is loaded—they are not loaded on demand, so there is no performance or memory savings involved in using include files.